The term “vector-borne diseases” is commonly used to describe diseases transmitted from an infected host to humans and animals by blood-feeding arthropods. Vectors of human diseases are typically species of blood-feeding mosquitoes and ticks that are able to transmit pathogens (viruses, bacteria or parasites) to humans and other warm-blooded hosts. Malaria, dengue fever, yellow fever and West Nile virus are among the most deadly infectious vector-borne diseases. The number of disease epidemics has dramatically increased in recent years not only in vector-borne human diseases, but also in vector-borne plant diseases.
The control of mosquitoes and other insect vectors of human, animal and plant pathogens is critical to solving various public health and economic problems. Although there have been decades of mosquito control efforts worldwide, mosquito-borne disease epidemics still remain for resurgence. Mosquito control methods have included chemical (insecticides) and biological (bacteria) techniques, habitat modification, integrated mosquito management and personal protection. One important method of reducing the risk of disease is through the use of repellents to protect people from being fed upon by arthropods. Repellents can be used as either topical repellents or spatial repellents.
Spatial repellents can be defined as repellents which are dispensed into the atmosphere of a three dimensional space and inhibit the ability of mosquitoes to locate and track a target such as humans or livestock (Nolen et al, U.S. No. 6,362,235). There has been much interest in recent years on using natural plant volatiles as repellents instead of synthetic repellents (Maia and Moore, 2011). However, from a practical application point of view, although there are devices in the marketplace to dispense such repellents, there are limitations to their effectiveness (Strickman, 2007). There are unrealized needs to provide mosquito and blood-feeding arthropod protection to humans and animals over much larger areas in open environments than possible at the present time. There are parallel needs for protection of plants against insect-vectored diseases.
As an example of a vector-borne plant disease, Huanglongbing (HLB) or citrus greening disease is a serious threat to cultivation of citrus crops. The Asian citrus psyllid, Diaphorina citri Kuwayama (D. citri), is the primary vector in citrus of the bacteria Candidatus liberibacter asiaticus and Candidatus liberibacter americanus. These bacteria are presumed to be responsible for HLB disease. The range of D. citri has expanded into citrus production areas throughout the world, threatening entire citrus groves on a regional scale, thereby making HLB one of the most serious threats to cultivation of citrus worldwide (Halbert and Manjunath, 2004).
Control of the Asian citrus psyllid is critical to the citrus industry. Current efforts to control D. citri populations primarily rely on application of broad-spectrum insecticides by area-wide aerial spraying programs. However, numerous applications of pesticides, often eight to ten times annually in Florida, cannot be indefinitely sustained. There are serious issues with D. citri developing pesticide resistance, the build-up of environment contaminants, the unintended elimination of natural biological control agents, and pesticide residues in citrus products.
The HLB situation is a crisis and cries out for new tools to control the D. citri population. Fortunately, there are potential attractive alternatives such as spatial repellents based on plant volatiles or plant-derived essential oils. Although there has been significant progress on repellent research in scientific laboratories, a practical repellent system (that includes both repellent and a delivery system) has not yet been developed. One key bottleneck in moving this research forward has been the lack of a slow-release device that maintains the volatile repellent above a behaviorally active threshold for extended periods of time as long as 150-200 days (Onagbola et al., 2011).
The art of emitting or releasing volatile compounds and substances has a long history. There are a variety of devices or systems described in the patent literature to evaporate or dispense volatile compounds or compositions into the atmosphere (Emmrich et al., U.S. Pat. No. 6,582,714; Kvietok, et al., U.S. Pat. No. 7,481,380). Materials commonly delivered include fragrances, deodorizers, disinfectants, insecticides, air fresheners, etc. Delivery dispensers typically can be categorized as “active dispensers” or “passive dispensers”. For example, a common active dispensing device is the aerosol spray dispenser that propels minute droplets of a volatile composition into the air. Active dispensers include various types of sprayers that operate by pressure, air displacement, or pump action. There are other dispensers that require an energy source. For example, devices or articles that dispense insecticide vapors often utilize the heating or burning of a liquid or solid substance to evaporate the active ingredients. Other dispensing methods include substrates such as paperboard or fabrics impregnated with volatile active ingredients, gelatinous materials that as they dry and shrink release a volatile compound into the air, and micro-encapsulated substances that achieve a slow release of volatile active ingredients. Evaporative surface (non-aerosol) devices typically utilize a wick or porous surface that provides a large surface area from which volatile liquid material can more quickly evaporate passively into the air. Attempts at improvements on the shortcomings of dispensing devices have included combining elements of both active and passive dispensers into a combined device.
Some vapor (or gas) dispensing devices have employed permeation membranes, but their intended usage generally has been focused on more specialized applications. For example, permeable membranes have been used in the production of calibration samples for gas or liquid analyzers, such as in tube devices (O'Keeffe, U.S. Pat. No. 3,412,935) and in devices having improved membrane permeability characteristics (Chand, U.S. Pat. No. 3,856,204). An apparatus used for the treatment of honeybee colonies for different honeybee diseases employed microporous membranes (Orth, U.S. Pat. No. 6,820,773). Vapor-permeable membranes also have found use in a fragrance product (Obermayer and Nichols, U.S. Pat. No. 4,356,969) and in a time-temperature indicator for monitoring the shelf lives of perishable articles (Patel, U.S. Pat. No. 4,195,058).
In an application for pest control, a sex pheromone has been dispensed by using a plastic bag through which the pheromone compound permeated the bag walls and was released as a vapor to the atmosphere (Kauth and Darskus, German Pat. No. 28 32 248; Kauth et al., German Pat. No. 29 45 655). Another approach to the control of a pest employed a capillary tubing of a polymeric material filled with a vaporizable substance, such as a pesticide, fungicide or sex pheromone, which permeated the tube walls and was released to the atmosphere; this dispensing body had good shape-retainability by integrating side-by-side a metal wire with the capillary tubing (Ohno, U.S. Pat. No. 4,600,146). A method for simultaneously controlling the rates of concurrent vapor release of two specific classes of sex pheromone compounds involved mixing in a unique proportion to achieve an overall solubility parameter and enclosing the liquid mixture in a permeable container such that two pheromone compounds permeated the wall and were dispensed into the atmosphere as a vapor (Yamamoto et al., U.S. Pat. No. 4,734,281).
The previously available methods, devices and systems were often limited to dispensing vapors within defined spaces, for example, a room or the area in the immediate vicinity of a device. These vapor dispensers are generally known in the art to provide inadequate effectiveness in larger, more open spaces, especially in large volumes of moving air, most particularly in the case of outdoor environments. Another difficulty is that they are not able to effectively dispense volatile compounds at a sustainable rate over very long periods of time. Furthermore, another undesirable characteristic is that there is often an initial burst of vapor followed by a continuous intensity decline, rather than a delivery of vapor at a rate substantially constant with time.
Additionally, there are commercial mosquito repellent devices utilizing spatial repellents that are marketed to provide protection in outdoor environments. A typical area of coverage is about 200-300 sq ft, but the protection claimed lasts for only about 4-18 hours. Such devices cannot be employed in applications for open environments such as young plantings in citrus groves and protecting humans in large outdoor facilities. These devices are not cost effective since the frequent replacement of repellent is too costly and too labor intensive.
Thus, there clearly is a need for effective systems and methods for the controlled delivery and continuous maintenance of volatile compounds in large open areas such as agricultural fields, citrus groves and orchards and structures such as large patios or decks and pavilions. However, there are serious challenges in the practical, effective deployment of volatile compounds in open environments. It is relatively simple to release volatile compounds. The current failure is maintaining an effective dynamic mixture of volatile compounds encompassing the object of protection for very long times. Therefore the design goals should include: (1) providing protection of objects of various geometrical shapes with specific spatial extent and height, (2) establishing effective concentrations under open outdoor conditions for defined volumes of volatile compounds, (3) maintaining the effective dynamic mixture in specific geometrically shaped volumes for very long periods of time, (4) providing continuous, predetermined and substantially constant release rates of volatile compounds from multiple sources, (5) sustaining release rates over extended time periods of protection up to and exceeding one year, (6) fine-tuning or adjusting release rates in the field as needed, and (7) developing systems that employ effective concepts to reduce waste and environmental impact.
The present invention meets these goals for the delivery of volatile compounds in a wide variety of practical applications. As one example, the present invention holds great promise to make available low-cost, highly effective mosquito protection systems utilizing plant-based spatial repellents to fight the impact of malaria, West Nile virus, dengue fever and other mosquito-borne diseases. As another example, the present invention will make effective biological strategies available in the fight to control the spreading of serious invasive plant diseases, such as the citrus greening disease (Zaka et al., 2010) and the potato zebra chip disease (Miles, 2010).
It is to the provision of systems for delivering and maintaining volatile compounds meeting these and other needs that the present invention is primarily directed.